CdI2 Crystals Research Articles

COMPARATIVE CRYSTAL FIELD ANALYSIS OF THE NI2+ ENERGY LEVEL SCHEME IN CdCl2, CdBr2, CdI2 CRYSTALS

The exchange charge model of crystal field was used to calculate the crystal field splitting of all Ni2+ energy levels (3d8 electron configuration) in three structurally similar crystals of CdCl2, CdBr2, and CdI2. The influence of the nature of the ligands and interionic distances on the values of the crystal field parameters and location of the split energy levels and relative intervals between them was identified. Comparison of the calculated energy levels with the experimental results available in the literature yielded good agreement.

Low-temperature luminescence of CdI2 under synchrotron radiation

Synchrotron radiation is applied to study visible and UV luminescence spectra and their excitation spectra of undoped as well as In and Sb doped cadmium iodide crystals at 10 K. The origin of principal luminescence bands and the role of impurities in the formation of emission centers are discussed. The luminescence properties have been explained based on the electronic structure of CdI2 crystals.

Morphology and Optical Properties of Nanostructures Formed in Non-Stoichiometric CdI2 Crystals

The morphology of nanostructures formed in non-stoichiometric CdI2 crystals has been studied, by using the atomic force microscopy methods. Morphological changes are observed, when the concentration of cadmium atoms approaches a non-stoichiometric threshold value of 0.1 mol%. The features in the phase composition of nanostructures are analyzed with the help of Raman and infrared absorption spectroscopies. The influence of the researched nanostructures on the optical characteristics of non-stoichiometric CdI2 crystals is analyzed.

Multiphoton excitation and thermal activation in indirect bandgap semiconductors

The single crystals of wide- and indirect-bandgap semiconductor CdI2 were grown and their optical properties as well as the defect-induced excitonic photoluminescence (PL) spectra were studied. The multiphoton excited PL spectra, charactering the emissions from excitons in the visible region, were taken. The PL intensity (IPL) was found to vary nonlinearly with pumping power (P). The IPL was also found to decrease with increasing temperature. The temperature dependence of IPL for each P followed a fashionable relationship, from which the activation energy (∆E) of the defect-induced excitonic trapping was calculated. The ∆E was found to slightly decrease with increasing P, with an average value of 2.5 meV. The results, however, demonstrates the existence of the self-trapped excitons responsible for the characteristics broad-band emission in the visible range. This study shows that the single crystals of CdI2 might have potential in applications as thermo-luminescent dosimeters in radiation measurements.

Long-term evolution of luminescent properties in CdI2 crystals

Fresh and aged melt-grown or gas-phase grown CdI2 crystals are studied by means of low-temperature photoluminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging. The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well as the changes in local surrounding of iodine atoms and the possible growth of polytypic modifications of CdI2 are taken into account when considering the diversity of optical spectra.

Formation of microtubes in CdI2 crystals doped with BiI3

The formation of microtubes (MTs) with rectangular cross section in the volume of CdI2 layered crystals doped with Bi3+ trivalent impurity ions has been observed for the first time by scanning electron microscopy. The MTs grow predominantly in pores that appear in the volume of CdI2-BiI3 crystals as a result of a difference in the valence of host Cd2+ ions and guest Bi3+ ions substituted into regular crystal lattice sites. The formation of MT structures is related to the rolling of I-Cd-I trilayers containing impurity ions, which takes place as a result of the presence of uncompensated charges at the edges. Deviation of the MT cross section from circular is explained by nonuniformity of the elastic properties of I-Cd-I trilayers with Bi impurity, which is related to the appearance of vacancies in the cation subsystem upon replacement of Cd2+ ions by Bi3+ ions.

Extrinsic luminescence centers in CdI2 crystals doped with PbI2 (10−4 to 1 mol %)

We have studied the luminescence properties of PbI2 doped (10−4 to 1 mol %) CdI2 crystals in the temperature range 4.2–90 K and those of 2H- and 4H-PbI2 crystals at 90 K. The results demonstrate that the Pb atoms produce impurity centers in the form of isolated Pb2+ ions and 4H-PbI2 nanocrystals located in the plane of the layers or along linear structural defects of CdI2. The formation of PbI2 nanocrystals was confirmed by atomic force microscopy. We discuss excitonic and electron-hole mechanisms of energy transfer to luminescence centers.

BiI3 nanoclusters in melt-grown CdI2 crystals studied by optical absorption spectroscopy

Optical absorption spectroscopy of CdI2–BiI3 layered crystals has been performed in the temperature range of 77–300K. The main absorption, observed in these crystals at 2.59eV is related to quantum confined exciton absorption of bimolecular BiI3 cluster. Low energy absorption peaks at 2.38eV, 2.06eV and 1.94eV can be tentatively assigned to larger clusters composed of 4, 6 or more BiI3 molecules. In ultra-thin samples of CdI2–BiI3 several bands at 2.84eV, 3.03eV and 3.27eV were observed which are ascribed to higher excited states of the excitons in bimolecular BiI3 cluster.

Carrier trapping and delocalization in PbI2-containing CdI2 crystals

Two pairs of electron and hole traps have been identified in PbI2-containing CdI2 crystals in the range 80–305 K using a combination of thermally stimulated and photoinduced depolarization of a photoelectret state. The electron traps are identified as Pb+ centers in nanocrystalline 2H- and 4H-PbI2 inclusions in the CdI2 lattice. One of the hole centers is assumed to be a Pb2+ vacancy. All of the traps in the binary crystalline system are related to the presence of PbI2 impurities as a consequence of the formation of heterojunctions between the CdI2 matrix and nanocrystalline PbI2 inclusions. We have calculated the energy distribution for filled traps.

Intensity Dependent Nonlinear Absorption in Direct and Indirect Band Gap Crystals under Nano and Picosecond Z-Scan

The nonlinear absorption properties of direct (GaN) and indirect (CdI2) band gap crystals have been studied by using an open aperture Z-scan technique under fundamental (1064 nm) and frequency doubled (532 nm) wavelength respectively with 10 ns or 60 ps pulse durations. Direct band gap crystal exhibits two and three photon absorption at all input irradiances. On the other hand, at low input irradiance the indirect band gap crystal exhibits saturable absorption (SA). At higher input irradiances two and three photon absorption becomes dominant. A monotonic increase of the nonlinear absorption coefficients with increasing laser pulse duration from 60 ps to 10 ns is observed for GaN and CdI2 crystals.

Thermo- and photostimulated processes of polarization and depolarization in CdI2: Ag

Currents of thermogradient polarization and depolarization of an electret state that arises in a photochromic crystal CdI2: Ag during one-side cooling of the sample in the dark from 325 to 90 K in the presence of a temperature gradient directed along the crystallographic axis C6 have been found. At 90 K, the crystal polarized in the thermogradient electromotive force field is characterized by the photosensitivity in the near-edge, impurity, and infrared spectral regions. It is revealed that the electret state in the CdI2: Ag crystal is also formed at room temperature during photolysis under irradiation of the samples by integrated light from a xenon lamp. Models of photosensitive centers formed upon doping of the CdI2 crystal from the melt by the Ag+ impurity and during the occurrence of thermo- and photostimulated chemical reactions are proposed. The mechanism of the photochromic effect, including the change in the charge state of silver impurity ions, is considered.

Cu-doping effects in CdI2 layered nanostructures: The role of photoinduced electron-phonon anharmonic interactions

The Cu-doping effects in CdI2 layered nanocrystals were studied experimentally. We used a photostimulated second harmonic generation (PSHG) in order to investigate the nanoconfined effects. It was found that the PSHG increased with increasing Cu-doping within a low-density limit. For the higher Cu content the PSHG output signal was decreased to the value observed for the undoped CdI2 crystal. The above results suggest that the Cu-metallic clusters play a crucial role in the processes responsible for the observed nonlinear optical effects. The large enhancement of the PSHG observed in the Cu-doped thin nanocrystals confirms the fact that the Cu-impurities of low concentrations favor stronger local photoinduced anharmonic electron-phonon interactions.

Spectral and kinetic characteristics of CdI2 and CdI2:Pb scintillators

We have studied the effect of lead dopant on the optical absorption, photoluminescence, and x-ray luminescence spectra, and the scintillation characteristics of CdI2 at room temperature. The crystals for the study were grown by the Stockbarger-Bridgman method. Activation of CdI2 from the melt by the compound PbI2 leads to the appearance in the absorption spectra in the near-edge region of an activator band at 395–405 nm, which is interpreted as an A band connected with electronic transitions from the 1S0 state to the 3P1 levels in the Pb2+ ion. For x-ray excitation, CdI2:Pb2+ crystals with optimal dopant concentration (∼1.0 mol%) are characterized by a light yield with maximum in the 570–580 nm region that is an order of magnitude higher than for CdI2 crystals in the 490–500 nm band. For α excitation, the radioluminescence kinetics for cadmium iodide is characterized by a very short (∼0.3 nsec) rise time and fast decay of luminescence, with τ1 ≈ 4 nsec and τ2 = 10–76 nsec. Depending on the conditions under which the crystals were obtained, the fast component fraction is 95%–99%. The crystal is characterized by a similar scintillation pulse in the case of excitation by x-ray pulses. The radioluminescence pulse shape for CdI2:Pb in the decay stage is predominantly exponential, with luminescence decay time constants τ1 ≈ 10 nsec and τ2 = 200–250 nsec. This system is characterized by low afterglow, at the level for the Bi4G3O12 scintillator. We have demonstrated the feasibility of using CdI2:Pb as a scintillator for detecting α particles.

Effect of Bi Doping on the Optical Properties of CdI2

The effect of Bi impurity on the optical absorption and roentgeno-, photo-, and thermoluminescence spectra of CdI2 crystals grown by the Bridgman-Stockbarger method is studied. CdI2 activation with BiI3 produces a number of absorption bands related to electronic transitions of Bi3+. Bi produces no emission centers in CdI2. The 550- to 560-nm luminescence of CdI2 is due to the radiative recombination of self-trapped anion excitons. The observed changes in spectral distribution and the reduction in light yield upon CdI2 activation with Bi3+ are mainly due to the reabsorption of emission from native centers by activator centers. Under x-ray excitation, CdI2: Bi3+ accumulates a small light sum at shallow trap levels related to native defects.

Low-temperature luminescent properties of Mn-activated layered cadmium and calcium halides

The luminescent properties of Mn-activated CdCl2, CdBr2, CdI2, and CaI2 crystals are studied in the range 4.2-200 K. The photo- and roentgenoluminescence spectra of the crystals show red emission bands due to the Mn activator. The excitation spectra and Mn luminescence kinetics are interpreted in terms of the crystal-field theory with consideration for electron paramagnetic resonance data. The red luminescence of the crystals at 4.2 K is attributed to the 4 T 1g(4 G) → 6 A 1g(6 S) electron-phonon transitions of Mn2+. The Racah parameters B and C are determined. The origin of the 600-nm luminescence in CdBr2:Mn2+ under excitation with a nitrogen laser and xenon lamp at 320-360 nm in the range 4.2–90 K is discussed.

One-, two-, and three-photon processes in CdI2 crystal

Abstract Single crystal of cadmium iodide (CdI2) was studied under laser excitation from the point of view of optical nonlinear materials having potential applications in electronic devices. Low temperature photoluminescence (PL) spectra of the CdI2 crystal were measured. To identify different photon absorption processes in the crystal, the dependences of the PL peak intensity (IPL) on excitation power (I0) were investigated. The dependences were found to follow approximately the power law IPL≈I0m, where m=1, 2 and 3 for N2, ruby and neodymium laser excitations respectively, indicating that the optical responses are due to a first-order, second-order and third-order processes respectively. Photoconductivity measurements in the crystal showed a quadratic dependence of the charge induced on incident laser power, giving evidence of two-photon induced conductivity. The majority carrier lifetime was also studied.

Polarized Infrared Absorption in CdI2:CN-Crystals

Cadmium halides (CdCl2, CdBr2 and CdI2) crystallize in a layered structure in which a Cd2þ-ion plane is sandwiched between two halogen ion planes. The fundamental layers pile up along the crystal c-axis to make up cadmium halide crystals. When doping a small amount of metal cyanide into these cadmium halide crystals, cyanic ions are supposed to be substituted for the halogen ions. In the present work, polarized infrared (IR) absorption has been investigated on CN centers doped in CdI2 crystals. For interpretation of these IR absorption, interaction of the CN elastic dipole with the local electric field in the crystal must be taken into account: A strong local electric uniaxial crystal field along the c-axis could arrange the CN molecular axis along this axis of the crystal. On the other hand, IR absorption and luminescence on CN centers in alkali halide crystals have been studied extensively without any indication of such arrangement of the dipole centers. The crystals were grown from melt in a vacuum sealed quartz ampule filled with CdI2 powder containing impurity Hg(CN)2 of 0.1mol% as an impurity. The CN -ion would substitute for a host I -ion. The specimens were cleaved in the size of 10 10 5mm from the CdI2 crystal ingot. The concentration of Hg(CN)2 in the specimens was not determined in the present study. The values of concentration in the following are those in the melt. The crystal samples were mounted on a copper holder of a closed-cycle He refrigerator and were cooled down to 6K. The present experiments were performed at the absorption and reflection spectroscopy station on the beamline BL43IR of the synchrotron radiation facility SPring-8, Hyogo, Japan. Polarized IR absorption spectra were measured by using a FT-IR spectrometer (Bruker IFS 120HR). The spectral resolution of this spectrometer was set for 0.025 cm 1 at 2000 cm . An internal globar lamp of the FT-IR spectrometer was used as the light source. Figure 1 shows the IR absorption spectra of CdI2:Hg(CN)2 (0.1mol%) in the range of 2060–2160 cm 1 for the polarization parallel (E k c) and perpendicular (E ? c) to the c-axis. Polarized light with E k c or E ? c was led on the polished crystal surface along the direction perpendicular to the crystal c-axis (E is the electric field vector of polarized light). The interference pattern due to BaF2 windows of the cryostat appears on the background of both spectra. Dichroism between the E k c and E ? c spectra is remarkable: All absorption peaks are observed only in the polarization of E k c. As described above, the CdI2 crystal has the uniaxial crystal field in the direction along the c-axis. The dichroic absorption results show that the CN -ions in the I -ion planes are arranged with their electric dipole axes parallel to the crystal c-axis. In addition, the full width at half-maximum (0.07 cm ) of the absorption lines is much narrower than those ( 5 cm ) in alkali halides. The rotational motion of CN molecules is restrained by the local uniaxial electric field along the c-axis in the CdI2 crystal. The local uniaxial electric field at halogen ion sites comes from Cd2þ-ion planes with positive charge and I -ion planes with negative charge. The absorption peaks are classified into two groups. The one is of sharp doublet peaks (2082.1, 2084.8 cm ) in the low energy region. These peaks are attributed to CN -ions simply substituted for host I -ions, which are called Isolated-CN centers [see Figs. 2(a) and 2(a0)]. They correspond to the absorption peak due to substitutional CN ions for I -ions in alkali-iodides, e.g., the peak observed at Fig. 1. The IR absorption spectra of CdI2 doped CN measured at 6K. Upper and lower spectrum is E k c and E ? c, respectively.

A combined optical, SEM and STM study of growth spirals on the polytypic cadmium iodide crystal

Some novel results of a combined sequential study of growth spirals on the basal surface of the richly polytypic CdI2 crystals by optical microscopy, scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) are presented and discussed. In confirmation of the known structural data, the STM pictures clearly reveal the value of unit cella(= b) dimension to be equal to 0.428 nm a and the value of the interaxial angle γ to be equal to 120°. Under the high resolution and magnification achieved in the scanning electron microscope, the growth steps of large heights seen in the optical micrographs are found to have a large number of additional steps of smaller heights existing between any two adjacent large height growth steps. When further seen by a scanning tunneling microscope, which provides still higher resolution, several unit substeps are seen to exist between two consecutive additional steps. The height of each substep is found to be equal to the unit cell height of the underlying polytype. The height of the larger additional steps are found to be integral multiple of the unit cell height. The horizontal distance between two consecutive growth steps is found to be an integral multiple of the lattice parametera. The interspacing of the growth steps of same step height is found to vary at different positions on the same crystal face.

Curious Variations in Energy Gap of Polytypic CdI2 and PbI2 Crystals on Laser Irradiation

Crystals of CdI2 and PbI2 were grown by zone melting. They were subjected to argon ion laser irradiation. Both time and intensity of radiation were systematically varied. The crystals of CdI2 were of type 4H while those of PbI2 were of type 2H. Their energy gaps were determined by UV spectrophotometry. The CdI2 crystals showed a linear variation of energy gap with intensity of radiation (0 to 125 mW), with the energy gap value decreasing from 3.28 to 3.11 eV, when the exposure time was kept fixed (7 min). When the time of exposure was varied, with the intensity of radiation fixed at 50 mW, the energy gap initially decreased from 3.28 to 3.07 eV and then gradually rose to be restored to the original value, thus showing a nonlinear variation. For the PbI2 crystals, the energy gap steadily decreased from 2.53 to 2.02 eV, when the intensity of exposure was varied. However, the value drastically decreased from 2.53 to 1.04 eV for the case of variation of exposure time. The results have been explained in terms of changes in phonon frequency, amount of absorption and anisotropy of thermal conductivity of the material.

Influence of weak atomic bonding on the height of growth spiral steps on cadmium iodide crystals

Step heights of growth spirals on CdI2 crystals, unambiguously measured for the first time by STM, do not turn out to be integral multiples of the unit cell height, as normally expected, unless enhanced by a constant value of 0.31nm. The discrepancy primarily arises from the weak van der Waals bonding between the neighbouring molecular sandwiches I–Cd–I constituting the structure and from the dangling bonds existing at the edge of a growth step and at the crystal surface. The result is significant for problems of surface physics and surface chemistry.